Hidden hotkeys in the 2.8
The gizmo category is not included in the keymap_hierarchy.py which can be found in release\scripts\modules\bl_keymap_utils. Therefore the category isn't displayed in the overview of the keymap. When you export the keymap you can view all shortcuts and it includes the ones for the gizmos.
It's not entirely clear to me why these entries are missing, hence I've created ticket T77799 on Blender's bug tracker. The reply by the UI team will be your definitive answer.